Solve the given system of equations.
$$\begin{array}{lr}x+y+3z=& 6\\ x+y+7z=& 18\\ x+9y+8z=& -19\end{array}$$

Select the correct choice below and fill in any answer boxes within your choice.
A. There is one solution. The solution set is , $◻,◻)\}$. (Simplify your answers.)
B. There are infinitely many solutions.
C. There is no solution.

Step 1 ：Subtract equation 1 from equation 2, we get: $4z=12$

Step 2 ：Solve for z: $z=\frac{12}{4}=3$

Step 3 ：Substitute $z=3$ into equation 1 and equation 2, we get: $x+y=6-3\ast 3=-3$ and $x+y=18-3\ast 7=-3$

Step 4 ：These two equations are the same, which means they are dependent.

Step 5 ：Substitute $z=3$ into equation 3, we get: $x+9y=-19-3\ast 8=-43$

Step 6 ：Subtract equation 4 from equation 6, we get: $8y=-40$

Step 7 ：Solve for y: $y=\frac{-40}{8}=-5$

Step 8 ：Substitute $y=-5$ into equation 4, we get: $x=-3-(-5)=2$

Step 9 ：So, the solution to the system of equations is $x=2$, $y=-5$, $z=3$

Step 10 ：$\boxed{{\displaystyle x=2,y=-5,z=3}}$